[+Marc for the arch timer]

On Fri, Feb 08, 2019 at 08:30:25PM +0100, Thomas Gleixner wrote:

On Fri, 8 Feb 2019, Thomas Gleixner wrote:

quoted

On Fri, 8 Feb 2019, Will Deacon wrote:

quoted

On Fri, Dec 07, 2018 at 05:53:21PM +0000, Will Deacon wrote:

quoted

Anyway, moving the counter read into the protected region is a little fiddly
because the memory barriers we have in there won't give us the ordering we
need. We'll instead need to do something nasty, like create a dependency
from the counter read to the read of the seqlock:

Maybe the untested crufty hack below, although this will be a nightmare to
implement in C.

How is the in kernel ktime_get() correctness guaranteed then?

Luck.

I think we'll have to introduce a dummy dependent stack read into our
counter accessor so that it's ordered by the smp_rmb(). Example diff
below, which I'll roll into a proper patch series later on.

Will

--->8

diff --git a/arch/arm64/include/asm/arch_timer.h b/arch/arm64/include/asm/arch_timer.h
index f2a234d6516c..bd55b4373700 100644
--- a/arch/arm64/include/asm/arch_timer.h
+++ b/arch/arm64/include/asm/arch_timer.h

@@ -150,8 +150,24 @@ static inline void arch_timer_set_cntkctl(u32 cntkctl)
 
 static inline u64 arch_counter_get_cntpct(void)
 {
+	u64 cnt, tmp;
+
 	isb();
-	return arch_timer_reg_read_stable(cntpct_el0);
+	cnt = arch_timer_reg_read_stable(cntpct_el0);
+
+	/*
+	 * This insanity brought to you by speculative, out-of-order system
+	 * register reads, sequence locks and Thomas Gleixner.
+	 *
+	 * http://lists.infradead.org/pipermail/linux-arm-kernel/2019-February/631195.html
+	 */
+	asm volatile("eor	%0, %1, %1\n"
+		     "add	%0, sp, %0\n"
+		     "ldr	xzr, [%0]"
+		     : "=r" (tmp)
+		     : "r" (cnt)
+		     : "memory");
+	return cnt;
 }
 
 static inline u64 arch_counter_get_cntvct(void)

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